[PATCH weston v3 4/4] gl-renderer: Emit GPU rendering begin and end timeline timepoints

[Alexandros Frantzis]

On Wed, Sep 27, 2017 at 01:18:47PM +0100, Chris Wilson wrote:
> Quoting Alexandros Frantzis (2017-09-27 13:09:16)
> > Use EGL fence sync objects to emit timepoints for the beginning and the
> > end of rendering on the GPU.
> 
> But why not use a qbo with GL_TIMESTAMP?
> 
> sync_file are only timestamped when they are signaled, which is very,
> very loosely coupled with the GPU rendering. Each will also force a
> flush and submission of the GL pipeline, which seems to me to be very
> heavyweight ?
> -Chris

Hi Chris,

using GL timestamp queries was my initial approach for implementing this
feature. However, Mesa currently lacks support for the
EGL_EXT_disjoint_timer_query extension so this approach was abandoned
(for now, at least) in favor of EGLSyncKHR + sync files.

Furthermore, sync files have the advantage that they can be easily
integrated with the event loop.  Query objects would lead to a more
complex flow since they require explicit polling for readiness.

My impression is that the sync file timestamps were "close enough" for
the intended purpose here. Another (minor) point is that the sync file
timestamps are in a clock domain (CLOCK_MONOTONIC) we can use directly
for correlation with compositor and other system events.

Finally, concerning the heaviness of the sync fence flushes, I don't
think any performance effects are significant enough to be a cause for
concern for our use case (but I guess it all depends on driver and GPU
details).

Thanks,
Alexandros